Manufacture of electron-emitting bodies



Patented Dec. 15, 1931 UNETED STATES PATENT QFFKIE ERNEST YEOMAN ROBINSON, 0F ENFIELD, ENGLAND, ASSIGNOR T0 ASSOCIATED ELEG- IRIGAL INDUSTRIES LIMITED, A BRITISH COMPANY MANUFACTURE OF ELECTRON-EMITTING BODIES No Drawing. Application filed November 30, 1928, Serial No. 322,971, and in Great Britain December -1, 1927.

This invention relates to the manufacture of electron emittin cathodes.

It is now believe that the major portion of the emission is due to the presence of the alkaline earth metal itself formed by reduction or electrolysis of the coating during operation of the device. The oxide layer may also be effective at certain temperatures for obtainin a high emission for a long period.

According to the present invention the core is heated in the metal vapour for a considerable time at such temperature that penetration or impregnation of the core by the metal occurs. Conveniently the core is immersed in alkaline earth metal powder and heated in a vacuum or an inert or reducing atmosphere to a high temperature whereby the metal vapour produced penetrates the core.

The cathode so impregnated may be assembled in a valve without further treatment, a definite oxidizing treatment being found to be unnecessary. It is believedthat the alkaline earth metal diffuses through the core metal forming at the surface of the core a monomolecular layer, and that oxygen or other gas necessary for high emission from the cathode is provided by the residual gases in the valve or gases absorbed by or absorbed on the electrodes.

Under conditions where a sufficient supply of alkaline earth metal by diffusion into the core is available, some of the residual oxygen or other electro-negative gas combines with the alkaline earth metal onthe cathode, an

alkaline earth metal monomolecular layer is produced on the cathode and the excess metal is volatilized. Hence the deeper the impregnation of the cathode core the longer the life of the device, or alternatively the higher may be the working temperature of the cathode and consequently the greater the emission. However, it is to be understood that the invention is not to be limited by any such theory.

It is preferred to employ barium as the activating metal and for the core a metal of comparatively low melting point, such for example, as nickel, since the impregnation thereof is in general easier for a lower melting point metal, but it is obvious that the core metal must not melt or have an appreclable volatllity at the operating temperature of the cathode of the order of 900 C. Descrlption will now be given of various methods of carrying out the invention.

A fabricated coreof nickel such as, for

example, a filament is first spooled on to a frame so that the turns of the filament are separated from each other and each loop is supported at as few oints as possible, in order that contact of t 1e filament wire with the supporting member will not result in an inferior coating of metal. ment is now cleaned and for this purpose a spool containing the filament may be heated under vacuum to say 1000 C. for half an hour. Alternatively, it may be heated in hydrogen.

When carbon is present in the filament it may be first heated in air and then in hydrogen, such process being repeated until the carbon is removed. The spool is thenv laid in a boat which may consist of nickel, and is closely packed with barium chips, filings, or powder. The boat is then placed in a vacuum oven and the temperature is raised and maintained until suificient impregnation of the filament core has occurred. Alternatively, the boat may be ovened in an inert or reducing atmosphere such as, for example, hydrogen. I In a still further modification the spool is packed with finely divided powder of the activating metal within a bomb of convenient size. The bomb after being closed is placed in a furnace, which ispreferably a hydrogen furnace, in order that the hydrogen may prevent contamination of the contents by difiusion through the heated Walls of the bomb. v

In variations of the above methods barium am'algarnmay be employed in the first in stance instead of barium and it is obvious that instead of barium metal, compounds or,

a mixture of materialswhich, when-heated 1n the impregnation process, Wlll evolve barium either by chemlcal dissociation or reaction, may be employed. For example,

barium hydride may be employed or barium The nickel filaazide may be employed, in which latter case only ovening under vacuum should be e1nployed owing to the danger of spontaneous decomposition.

The temperature and the time of heating may be varied within very Wide limits depending on the size of the core, the nature of the metal of the core, the nature of the impregnating metal and the degree of impregnation required. vW'ith a nickel filament of diameter 0.02 mm., impregnation has been found to be satisfactory-if the boat is heated at about 700 C. to 800 C. for half an hour. 7

\Vhen the charge is heated in hydrogen or in a bomb higher temperatures may be conveniently employed, for example the bomb may be heated at about 1000 C. to 1100 C. for between one-half to two hours. After impregnation the filament may be unwound from the spool and mounted in Valves.

As stated heretofore it is unnecessary to apply a definite oxidizing process to the cathode. The cathode may therefore, be mounted into the valve and the valve immediately pumped. During or after pumping the anode is heated preferably by high frequency currents to free it from occluded gases, and a getter such as magnesium, misch metal or barium is vaporized within the device. The valve is then operated under slightly overload conditions for about two hours to bring the emission up to steady value.

Additionally if desired, the filament may be flashed at as high a temperature aspossible for aperiod between 5 and 30 seconds prior to the aging. This process of flashing before aging tends to give more consistent emission from different values.

Although it is found unnecesary to give the filament a definite oxidizing treatment, it is believed that oxygen or other electro negative gas is necessary to obtain very high emission. If such is the case then the oxygen is no doubt supplied in the case considered above by the gases evolved from the electrodes, or left in the device after pumping.

The filament may be given a definite oxidizing treatment if desired by flashing it in air before pumping or by heating it in the valve before cleaning up the residual gases with the metallic getter.

I claim as my invention The process of manufacturing an electronemitting cathode which comprises placing in a receptacle a fabricated core consisting of one or more elements of the platinum and nickel groups, packing barium around said body, and heating the contents of said receptacle sufficiently to cause saidbarium to impregnate said body.

In testimony whereof I have hereunto subscribed by name this sixteenth November, 1928. r

ERN *ST YEOMAN ROBINSON.

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